This invention relates to picture information processing and storing devices used for picture information processing apparatus such as document filing apparatus.
Recently, there have been known document filing apparatus for preserving great quantities of document picture information such as information concerning characters and drawings by obtaining electric signals through photoelectric conversion of light signals, which are obtained by scanning the documents or the like in optical two-dimensional scanning, and progressively storing or filing these electric signals in a large capacity storage device. The large capacity storage devices that are used for such document filing apparatus have inherent writing and reading speeds. The reading and writing speeds of the storage devices are often higher than the operation speed of the picture information reading apparatus, which generate electric picture signals by optically scanning documents. In order to obtain smooth transfer of information between the reading unit and storage unit, therefore, a buffer memory or page buffer capable of storing a unit quantity of document picture information, for instance for one page, is usually necessary.
The document picture information read out from a picture information reading device usually includes much redundancy, and directly storing it in the memory results in inferior utility factor of the memory. Accordingly, it has been a practice to code the readout document picture information by using a redundancy suppression system or high efficiency coding system such as MH (Modified Huffman) coding system which is accepted to comply with the international standards by CCITT (International Telegraph and Telephone Consultative Committee) and is used in the fields of facsimile, and store the coded picture information thus obtained by high efficiency coding in a storage device. By so doing, as great quantity of picture information as several ten times that in case of directly storing document picture information can be stored in a memory device having the same memory capacity to obtain an extremely increased memory utility factor.
The MH coding system will be briefly described here. This system uses a one-dimensional run length coding system, in which 1,728 bits, for instance, are used to represent picture information in one scanning line of an A4 size short side. The white or black run lengths contained in one scanning line are each represented by a plurality of successive "0" or "1" bits. With the ordinary document, the average run length of black run often appearing is 0.3 to 0.6 mm, while the average run length of white run often appearing is 1 to 2 mm. In the MH coding system, bits are assigned on the basis of the probability of the white and black run length occurrence; less bits are assigned to run lengths of higher occurrence probabilities, and more bits to run lengths of lower occurrence probabilities. Also, a basic code called a terminating code (T) is solely used for run lengths of 0 to 63 bits, while for run lengths of 64 to 1,728 bits a make-up code represented by 64.times.M (M being a positive integer) is used together with the terminating code (T). More particularly, 64 different MH terminating codes are assigned to white and black run lengths of respectively 0 to 63 bits, and M different make-up MH codes are assigned to white and black run lengths of respective 64.times.i (i being a positive integer in a range of 1.ltoreq.i.ltoreq.M) bits. For example, a terminating code "10011" is assigned to a white run length of 8 bits, and a terminating code "000101" is assigned to a black run length of 8 bits. MH coding and decoding can thus be realized by using a ROM, in which the relation between the available run lengths and white and black terminating codes and make-up codes is stored. The MH coding and decoding system is explained in detail in the paper "SPECIAL RAPPORTEUR FOR GROUP 3 EQUIPMENT" reported by CCITT (September 1976).
In the prior art document filing apparatus, the original picture information read out line after line from document by the picture information reading device is stored sequentially line after line in a page buffer. When the original picture information for one page has been stored in the page buffer, it is then sequentially supplied from the page buffer to an MH coding and decoding circuit for conversion into MH code, which is then transferred to a main recording unit including a magnetic tape or disc. The original picture information is stored in the page buffer prior to MH coding in order to permit display or printing of the original picture information on the basis of the stored content in the page buffer.
In the MH coding, the original picture information compression factor varies greatly depending upon the content of the original picture information for the line scanned by the main scanning of the reading device, i.e., depending upon the various white and black run lengths; in an extreme case where the document picture information of one scanning line is solely constituted by a single white run length the compression factor is 1/100. Therefore, in order to write MH coded picture information into the main storage device having a fixed writing and reading speed f (bits per second) without use of any dummy code, it is necessary to use a high speed MH coding unit by setting the reading speed of the page buffer to 100.times.f (bits per second) or above. However, this requirement is difficult to meet. Usually, therefore, a buffer memory is used for temporarily storing the MH coded picture information from the page buffer. The storage capacity of the buffer memory may be small when storing picture information obtained by MH coding with a high compression factor, but it has to be large when storing MH coded picture information at a low compression factor, for instance MH coded picture information obtained from picture information representing short white and black run lengths alternately occurring many times in one scanning line. For example, for storing read-out picture information obtained by scanning a document having a size of 200 mm.times.300 mm that 8 bits/mm are scanned in the X direction while the document is relatively moved in Y direction by 1 mm with a resolution of 8 lines/mm, a page buffer having a capacity of 200.times.300.times.8.times.8 bits (=3.84 megabits) is necessary. The buffer memory for temporarily storing the MH coded picture information from the page buffer is required to have a storage capacity equal to about one half the storage capacity of the page buffer. Where the buffer memory having such storage capacity is constructed with an IC memory or the like, however, it is inevitable that the entire circuitry is extremely large. In addition, the writing and reading operations for the buffer memory have to be executed simultaneously. This means that the write and read controls must be separately executed and thus complicates the control operation as a whole.